1. Field of the Invention
This invention is related to a composite solder and a solder joint comprising same. The composite solder is in the form of a solder preform. A solder preform is solder which has been shaped into a two-dimensional shape such as a sheet or ring. A composite solder is a solder in the form of a composite material.
2. Description of the Prior Art
Solder preforms are used to join various parts of an electronic package. For example, they are used to join a ceramic cap and a multilayer ceramic substrate in a multichip module. Because the thermal expansion coefficient of a solder is in general much higher than that of a ceramic, the solder joint suffers from a poor resistance to thermal fatigue. Thus, there is a need for a solder with a low coefficient of thermal expansion.
Composite materials can be tailored to exhibit a chosen thermal expansion coefficient, as the filler species and the filler volume fraction can be judiciously chosen. For a low thermal expansion composite, the filler must have a low thermal expansion coefficient. Moreover, the filler should preferably be a good electrical conductor, because the soldered joint may serve as an electrical connection as well as a mechanical connection. It should also preferably be a good thermal conductor for heat dissipation from the electronic package. In addition, the filler should be wetted well by liquid solder in order to facilitate composite fabrication A further requirement is that, upon remelting and solidification of the solder in the solder-matrix composite, the filler distribution in the composite remains uniform.
Molybdenum particles have been used as a filler in solder-matrix composites, as for example described in IBM Technical Disclosure Bulletin 29(4), 1573 (1986). However, their distribution in the solder (Sn-40% Pb alloy) became non-uniform after remelting and solidification of the solder in the composite. Thus, molybdenum is not a suitable filler in spite of its low coefficient of thermal expansion.
Carbon fibers have nearly zero thermal expansion coefficient. In contrast to ceramic fibers (which also have low thermal expansion coefficients), carbon fibers are a good electrical conductor and a good thermal conductor; however, the effect, if any, of carbon fibers in a solder preform have not been properly investigated.
Tin-lead alloys are most commonly used for solders. Carbonfiber reinforced tin-lead alloys had been previously fabricated by liquid metal infiltration for bearing applications, [C.F. Old, I. Barwood and M.G. Nicholas, Pract. Met. Compos., Spring Meet., B47-B50. London, Engl., Inst. Metall. (1974)] and by investment casting for fundamental process study [A. Miyase and K. Piekarski, Adv. Res. Strength Fract. Mater., Int. Conf. Fract., 4th, Meeting Date 1977, Volume 3B, 1067-71. Edited by David M.R. Taplin. Elmsford, NY, Pergamon, 1978]. For both methods, the carbon fibers were electroplated with copper in order to ensure good wetting of the fibers by the alloy. Carbon-fiber reinforced tin (containing 7.5 wt. % Sb and 3.5 wt. % Cu) have also been fabricated by squeeze casting for fundamental process study wherein the carbon fibers were electroplated with nickel in order to avoid the oxidation of the fibers during casting.
It was not, however, recognized that such carbon fiber reinforced materials would have particularly good utility in the formation of solder joints.